Methods for detecting combustion misfires are utilized in spark-ignition engines in order to, on the one hand, detect an uneven engine running and to minimize the uneven running by suitable control arrangements while, on the other hand, and above all, to avoid a deterioration of exhaust-gas values and to protect the exhaust-gas catalytic converter. The rough running of the engine can, for example, be caused by valve coking or, in direct injecting engines, by quality scattering of the characteristic values of the injection valves. The deterioration of the exhaust gas values is caused by misfires as a consequence of an engine defect. If an engine defect is detected, which is apparent because of a misfire, then the driver can, for example, be advised thereof by a warning lamp in order to initiate a repair as rapidly as possible.
Such methods utilize the realization that a combustion which does not take place within a cylinder of an engine causes characteristic changes of the trace of the torque of the engine compared to normal operation. From the comparison of the torque traces, one can distinguish between normal operation of the engine without misfire and so-called misfire-burdened operation. A misfire-burdened operation of one or several cylinders contributes to the total torque trace of the engine with a reduced contribution. This contribution can be determined by detecting the actual torques of the cylinders via an evaluation of the time-dependent trace of the crankshaft rotation or the camshaft rotation. According to the species method, a crankshaft angular region is assigned to a specific region of the piston movement of each cylinder or several simultaneously ignited cylinders. The crankshaft angle region is characterized as a segment. The segments, which belong to each cylinder or to the cylinders, are, for example, realized by markings on a transducer wheel coupled to the crankshaft. The segment time is the time in which the crankshaft passes over the corresponding angular region of the segment and is essentially dependent upon the energy converted in the combustion stroke. As a consequence of defective torque contributions, misfires lead to an increase of the ignition-synchronously detected segment times which are determined for each segment by scanning the markings on the transducer wheel by a suitable sensor. The more even the engine runs, the less will be the differences between the individual segment times. As already known from the state of the art, for example, from DE-OS 4,138,765, an index for the rough running of the engine is computed from the differences of the segment times. Additional boundary conditions are mathematically compensated and are, for example, the increase of the engine rpm for a vehicle acceleration. The rough running value is computed for each ignition of a cylinder or of simultaneously igniting cylinders. In a subsequent method step, the rough running value is compared ignition-synchronously in a desired value comparator to a threshold value. If the determined rough running value exceeds the threshold value, then this is evaluated as a misfire of the affected cylinder. The threshold value is, if needed, dependent upon operating parameters such as load and rpm. If misfires are determined, then their number is added. When a specific misfire rate is exceeded then, for example, a suitable warning device indicates this unevenness.
The described method from the state of the art has proven itself principally for internal combustion engines having single igniting cylinders. In internal combustion engines having several simultaneously igniting cylinders, the problematic results that for a misfire detection for a crankshaft segment wherein, for example, two cylinders ignite simultaneously, no detection can take place as to which of the two cylinders has combustion misfires. Furthermore, for internal combustion engines having several simultaneously igniting cylinders, the problem is present that, for misfire-burdened operation of one of these cylinders, the drop in torque caused thereby is present and is measured at the crankshaft but, in contrast to the misfire of a single igniting cylinder, the torque reduction is considerably less because the other cylinder exhibits a normal combustion operation.
This has the consequence that, when comparing the rough running values of crankshaft segment regions having several simultaneously igniting cylinders, misfires, under certain circumstances, cannot be detected because of the threshold value which is necessarily set higher for a detection with an individual ignition.